When ablative energy, e.g., high intensity focused ultrasound (HIFU) energy, radiofrequency (RF) energy, microwave energy and/or laser energy, is applied to tissue, significant physiological effects may be produced in the tissue resulting from thermal and/or mechanical changes or effects in the tissue. Thermal effects include heating of the tissue; and, when the tissue is heated to a sufficiently high temperature, tissue damage such as coagulative necrosis can be produced. In order to produce thermal effects in tissue, ablating members, e.g., ultrasound emitting members such as transducers or electrodes, have been used to emit ablative energy which is applied to tissue. For example, ablating members may by positioned adjacent or in contact with the tissue or by coupling the ablating members to the tissue via a coupling medium, stand-off and/or sheath. By focusing the ablating energy at one or more specific focusing zones within the tissue, thermal effect can be confined to a defined location, region, volume or area. Depending on the type of ablative energy used, the location, region, volume or area that is ablated may be remote from the ablating member.
With the use HIFU, one or more focusing zones at or within a designated target location, region, volume or area within a larger mass, body or area of tissue can be subjected to high intensity ultrasound energy while tissue surrounding the target area is subjected to much lower intensity ultrasound energy. In this manner, tissue in the target area can be heated to a sufficiently high temperature so as to cause a desired thermal effect such as tissue damage, ablation, coagulation, denaturation, destruction or necrosis while tissue surrounding the target area is not heated to damaging temperatures and, therefore, is preserved. Heating of tissue in a target location, volume, region or area to an ablative temperature creates an ablative lesion in the tissue in the target location, volume, region or area that is desirable in the treatment of various medical conditions, disorders or diseases. For example, the lesion may remain as tissue having altered characteristics or may be naturally degraded and absorbed by the patient's body and thusly eliminated such that the remaining body, mass or area of tissue is of smaller volume or size due to the absence of the ablated tissue.
The use of HIFU to eliminate tissue or to alter the characteristics of tissue in a target location, volume, region or area within a larger mass, body or area of tissue presents many advantages including minimization of trauma and pain for the patient, elimination of the need for a surgical incision, stitches and exposure of internal tissue, avoidance of damage to tissue other than that which is to be treated, altered or removed, lack of a harmful cumulative effect from the ultrasound energy on the surrounding non-target tissue, reduction in treatment costs, elimination of the need in many cases for general anesthesia, reduction of the risk of infection and other complications, avoidance of blood loss, and the ability for high intensity focused ultrasound procedures to be performed in non-hospital sites and/or on an out-patient basis.
The action of the heart is known to depend on electrical signals within the heart tissue. Occasionally, these electrical signals do not function properly, thereby causing heart arrhythmias. Heart arrhythmias, such as atrial fibrillation, have been treated by surgery. For example, a surgical procedure called the “Maze” procedure was designed to eliminate atrial fibrillation permanently. The procedure employs incisions in the right and left atria which divide the atria into electrically isolated portions which in turn results in an orderly passage of the depolarization wave front from the sino-atrial node (SA node) to the atrial-ventricular node (AV node) while preventing reentrant wave front propagation. Although successful in treating AF, the surgical Maze procedure is quite complex and is currently performed by a limited number of highly skilled cardiac surgeons in conjunction with other open-heart procedures. As a result of the complexities of the surgical procedure, there has been an increased level of interest in procedures employing ultrasound devices or other types of ablation devices, e.g. thermal ablation, microwave ablation, RF ablation, cryo-ablation or the like to ablate tissue along pathways approximating the incisions of the Maze procedure. Electrosurgical systems for performing such procedures are described in U.S. Pat. No. 5,916,213 to Haissaguerre, et al., U.S. Pat. No. 5,957,961 to Maguire, et al. and U.S. Pat. No. 5,690,661, all incorporated herein by reference in their entireties. Procedures are also disclosed in U.S. Pat. No. 5,895,417 to Pomeranz, et al, U.S. Pat. No. 5,575,766 to Swartz, et al., U.S. Pat. No. 6,032,077 to Pomeranz, U.S. Pat. No. 6,142,994 to Swanson, et al. and U.S. Pat. No. 5,871,523 to Fleischman, et al., all incorporated herein by reference in their entireties. Cryo-ablation systems for performing such procedures are described in U.S. Pat. No. 5,733,280 to Avitall, also incorporated herein by reference in its entirety. High intensity focused ultrasound systems for performing such procedures are described in U.S. Patent Application Publication No. 2005/0080469 to Larson et al. and U.S. Pat. No. 6,858,026 to Sliwa et al., U.S. Pat. No. 6,840,936 to Sliwa et al., U.S. Pat. No. 6,805,129 to Pless et al. and No. 6,805,128 to Pless et al., all incorporated herein by reference in their entireties.
High intensity focused ultrasound is an attractive surgical ablation modality as the energy can be focused to create heat at some distance from the ablating member (or transducer). In epicardial applications, most of the heat loss is to the blood, which is also some distance from the transducer. This is in contrast to most other technologies, in which heating occurs close to the ablating element (or electrode) and deeper heating is by thermal conduction. Additionally, since the coronary arteries are typically towards the epicardial surface, they are theoretically less susceptible to heating and subsequent constriction by a device such as a HIFU device, which can generate heat deep within the myocardium. For example, a non-irrigated RF epicardial ablation approaches has the highest heating occurring at the epicardial surface. Any transfer of heat to the deeper endocardium is by thermal conduction. Irrigated RF epicardial ablation approaches allow the heat to penetrate deeper into the tissue, but tend to be limited in depth. In contrast, a HIFU approach can focus the energy to generate heat deeper within the tissue at a substantial distance from the transducer.
Another therapeutic method to terminate AF is to ablate an area that is sufficiently large enough such that there is not enough critical mass to sustain the reentrant waveform characteristic of the arrhythmia.
In conjunction with the use of ablation devices, various control mechanisms have been developed to control delivery of ablation energy to achieve the desired result of ablation, i.e. killing of cells at the ablation site while leaving the basic structure of the organ to be ablated intact. Such control systems may include measurement of temperature and/or impedance at or adjacent to the ablation site, as are disclosed in U.S. Pat. No. 5,540,681 to Struhl, et al., incorporated herein by reference in its entirety.
Additionally, there has been substantial work done toward assuring that the ablation procedure is complete, i.e. that the ablation extends through the thickness of the tissue to be ablated, before terminating application of ablation energy. This desired result is some times referred to as a “transmural” ablation. For example, detection of a desired drop in electrical impedance at the electrode site as an indicator of transmurality is disclosed in U.S. Pat. No. 5,562,721 to Marchlinski et al., incorporated herein by reference in its entirety. Alternatively, detection of an impedance rise or an impedance rise following an impedance fall are disclosed in U.S. Pat. No. 5,558,671 to Yates and U.S. Pat. No. 5,540,684 to Hassler, respectively, also incorporated herein by reference in their entireties.
Sometimes ablation is necessary only at discrete positions along the tissue. This is the case, for example, when ablating accessory pathways, such as in Wolff-Parkinson-White syndrome or AV nodal reentrant tachycardias. At other times, however, ablation is desired along a line, called linear ablation. This is generally the case for atrial fibrillation, where the aim is to reduce the total mass of electrically connected atrial tissue below a threshold believed to be critical for sustaining multiple reentry wavelets. Linear lesions are created between electrically non-conductive anatomic landmarks to reduce the contiguous atrial mass.
Various approaches have been employed to create elongated lesions using ablation devices. The first approach is simply to create a series of short lesions using an ablating member, e.g., an electrode, moving it along the surface of the organ wall to be ablated to create a linear lesion. This can be accomplished either by making a series of lesions, moving the ablating member between lesions or by dragging the ablating member along the surface of the organ to be ablated and continuously applying ablation energy, e.g., as described in U.S. Pat. No. 5,897,533 to Mulier, et al., incorporated herein by reference in its entirety.
A second approach to creation of elongated lesions is simply to employ an elongated ablating member, e.g., an elongated electrode, and to place the elongated ablating member along the desired line of lesion along the tissue. This approach is described in U.S. Pat. No. 5,916,213, cited above. A third approach to creation of elongated lesions is to provide a series of ablating elements, e.g., a series of spaced-apart band or coil electrodes, and arrange the series of ablating elements along the desired line of lesion. After the ablating portion of the ablation device has been properly positioned, the ablating elements are energized simultaneously or one at a time to create the desired lesion. If the ablating elements are close enough together the lesions can run together sufficiently to create a continuous linear lesion. Electrodes that may be activated individually or in sequence, are disclosed in U.S. Pat. No. 5,957,961, also cited above. In the case of multi-ablating element devices, individual feedback regulation of ablated energy applied via the ablating elements may also be employed.
A method used for guidance for various medical devices in minimally invasive and robotic surgery, e.g., cardiothoracic surgery, is that of endoscopic visualization. Images from endoscopic light guides and cameras placed within a patient's body, e.g., the patient's thoracic cavity, may be displayed on a video monitor that is viewed by a surgeon. The effective use of an endoscopic visualization method depends on there being sufficient open space within the working area of the body. Various retractors and tissue spreading instruments are sometimes used to hold tissues away from the working field within the body. Pressurized gas or gasses may be introduced into the thoracic cavity to help create space in which to work with a sufficient field of view. A lung may be deflated to drop it away from the working field. Without sufficient space and field of view, it is difficult for the surgeon to recognize the anatomical location and identity of structures viewed on the video display. The requirement for space surrounding the working field has the effect of limiting the regions which can be safely and confidently accessed by minimally invasive endoscopic techniques. For example, it is very difficult for the surgeon to endoscopically visualize the passage of instruments through the spaces posterior to and around portions of the heart such as the transverse and oblique sinuses. Due to these limitations, some procedures have not been attempted using minimally invasive endoscopic techniques. Other methods or techniques that may be used for guidance or navigation of various surgical instruments in minimally invasive medical procedures, include electromagnetic methods, electric field methods and ultrasound methods.